1. Field of the Invention
This invention relates to ships' steering systems or helms and, more particularly, relates to a variable ratio helm in which the ratio of rudder rotation to helm rotation is non-linear.
2. Description of the Prior Art
Traditionally a ship's steering system or helm has required a plurality of complete (360.degree.) rotations of the helm's wheel to effect the hardover rudder rotation, i.e., a full rudder rotation of about 45.degree.. This insensitivity of rudder rotation or movement to helm wheel rotation is an effect which grew out of the mechanical advantage of block and tackle coupling of the helm to the rudder which permitted a seaman to effectively generate enough force at the helm to effect a change of heading of a large ship by substantial displacement of the rudder, which is subjected to extremely large fluid forces. This insensitivity of rudder rotation to helm rotation, however, made heading keeping, that is, maintaining the ship aligned on course, much easier because large helm rotations, as large as 60.degree., in either direction resulted in minor repositioning of the rudder that is on the order of 5.degree. in either direction and accordingly, less stringent control of the helm was required to keep the ship on course. It is to be noted that hardover rudder varies from ship to ship but is on the order of 45.degree. rudder rotation from midship. Accordingly, as various mechanical and electrical systems eventually replaced the manual-pulley power arrangement for operation of the rudder, the insensitivity of the rudder to helm rotation remained, i.e., a plurality of helm rotations for hardover to hardover rudder, in order that rudder rotation during heading keeping remained insensitive to large wheel rotations as in the previous known pulley arrangements. However, for change of heading the insensitivity of the rudder to the rotation of the ship's steering system required a plurality, generally about three complete rotations of the helm from hardover to hardover rudder which can be a troublesome maneuver and which in an emergency situation requires additional time to order. Furthermore, during an emergency situation the seaman at the helm would order a hardover rudder, which as stated above, takes some time to rotate the wheel to this position, and if no response was forthcoming would then have to generate an emergency or Non Follow Up (NFU) operation to rotate the rudder in the required direction.
Therefore, there is a need to provide a helm which is insensitive to large rotations about midships for heading keeping and, also, highly responsive to helm rotation for change of heading. Furthermore, in this respect, during an emergency situation the instinctive reaction of the seaman at the helm to continue turning the wheel towards and past the hardover rudder position dictates a need to provide an emergency Non Follow Up system incorporated in the helm order device at the hardover rudder position.